The Nature of Light

• As a wave…– A small disturbance in an electric field

creates a small magnetic field, which inturn creates a small electric field, and soon…

• Light propagates itself “by its bootstraps!”– Light waves can interfere with other light

waves, canceling or amplifying them!– The color of light is determined by its

wavelength.

• As a particle…– Particles of light (photons) travel through

space.– These photons have very specific energies.

that is, light is quantized.– Photons strike your eye (or other sensors)

like a very small bullet, and are detected.

• Light is radiant energy.• Travels very fast –

300,000 km/sec!• Can be described either

as a wave or as aparticle travelingthrough space.

The Effect of Distance on Light

• Light from distant objectsseems very dim– Why? Is it because the photons

are losing energy (Tired Light)?– No – the light is simply

spreading out as it travels fromits source to its destination

– The farther from the source youare, the dimmer the light seems

– We say that the object’sbrightness, or amount of lightreceived from a source, isdecreasing

2d4

OutputLight TotalBrightness

!=

This is an inverse-square law – the brightness decreases as the square of the distance (d) from the source

The Nature of Matter

• The atom has a nucleus atits center containing protonsand neutrons

• Outside of the nucleus,electrons whiz around inclouds called orbitals– Electrons can also be

described using wave orparticle models

– Electron orbitals are quantized– that is, they exist only atvery particular energies

– The lowest energy orbital iscalled the ground state, oneelectron wave long

• To move an electron from one orbital to thenext higher one, a specific amount of energymust be added. Likewise, a specific amountof energy must be released for an electron tomove to a lower orbital

• These are called electronic transitions

The Chemical Elements

• The number of protons (atomic number) in a nucleusdetermines what element a substance is.

• Each element has a number of electrons equal to thenumber of protons

• The electron orbitals are different for each element,and the energy differences between the orbitals areunique as well.

• This means that if we can detect the energy emitted orabsorbed by an atom during an electronic transition,we can tell what element the atom belongs to, evenfrom millions of light years away!

Absorption

• If a photon of exactly theright energy(corresponding to theenergy differencebetween orbitals) strikesan electron, that electronwill absorb the photonand move into the nexthigher orbital– The atom is now in an

excited state• If the photon is of higher

or lower energies, it willnot be absorbed – it willpass through as if theatom were not there.

• This process is called absorption• If the electron gains enough energy to

leave the atom entirely, we say theatom is now ionized, or is an ion.

Emission

• If an atom dropsfrom one orbitalto the next lowerone, it must firstemit a photonwith the sameamount of energyas the orbitalenergydifference.

• This is calledemission.

Light

• In Astronomy, it is far too difficult to visitstars and most planets in person

• Astronomers’ primary tool in learning aboutthe universe is electromagnetic radiation, orlight

• The colors that the human eye can see definethe visible spectrum, but there is much more tolight than this narrow band of color!

Wavelength

• The colors we see are determined by thewavelength of light.

• Wavelength is the distance betweensuccessive crests (or troughs) in anelectromagnetic wave.

• This is very similar in concept to thedistance between the crests in oceanwaves!

• We denote the wavelength of light by thesymbol λ.

• Wavelengths of visible light are verysmall!– Red light has a wavelength of 7×10-7

meters, or 700 nanometers (nm)– Violet light has a wavelength of 4×

10-7 meters, or 400 nm– Colors in between red and violet

(remember ROY G BIV?) haveintermediate wavelengths

Frequency

• Frequency and wavelength are related by:

c=!"#‘c’ is the speed of light.

!

" =c

#

White Light

• Light from the Sunarrives with allwavelengths, and weperceive this mixture ofcolors as white

• Newton demonstratedthat white light could besplit into its componentcolors with a prism, andthen recombined intowhite light with a lens

The Electromagnetic Spectrum I

• There is more to light than just thevisible part of the spectrum– Radio waves are very long

wavelength photons (notsound!) with wavelengthslonger than a meter or so

– Microwaves (yes, the ones wecook with) are at the upperend of the radio part of thespectrum

– Infrared wavelengths are justlonger in wavelength than thevisible spectrum

The Electromagnetic Spectrum II

• Above the visible…– Ultraviolet waves are just

shorter in wavelength thanvisible waves. These arethe waves that tan or burnus!

– X-rays come mostly fromstellar sources, and canpenetrate many materials,like skin, muscle and bone

– Gamma rays have theshortest wavelength

Energy Carried by Photons

• A photon carries energy with itthat is related to its wavelength orfrequency

• From this we see that longwavelength (low frequency)photons carry less energy thanshort wavelength (high frequency)ones. This is why UV waves giveus a sunburn, and X-rays let uslook through skin and muscles!

!

E =h " c

#= h "$

h=Planck's constant = 6.626068 × 10-34 m2 kg / s

Types of Spectra

Spectra of Astronomical Objects